Known means for influencing the flow conditions include, for example, swirl control valves, which are located within a cylinder inlet passage having in each case two inlet valves at the outlet end. Each swirl control valve can be brought by a control member controllable by the electronic engine control unit into a starting position substantially parallel to or into an effective position transverse to the main direction of flow in the inlet passage.
In the effective position the flow conditions are influenced so as to make more stable combustion possible, particularly at lean air/fuel ratios. By selective use of such a swirl control valve the proportion of fuel in a lean engine mixture can be decreased without causing combustion instabilities such as misfires. This is desirable with regard to decrease in the nitrogen oxides emissions, since in principle the leaner the engine is operated, the lower is the emission of nitrogen oxides.
However, in the effective position the swirl control valve has a relatively high resistance to flow. Consequently flow losses occur which limit the maximum air/fuel throughput into the cylinder. Hence at higher rotational speed/torque requirements it is necessary to move the swirl control valve from the effective position into the starting position in order to obtain an adequate throughput of air. It is further known to operate the internal combustion engine using a stoichiometric to rich mixture at relatively high rotational speed or torque requirements such as occur, for example, under full load conditions or in the case of acceleration of the vehicle, since in this case the torques obtainable in lean burn operation are inadequate.
For these reasons in known methods the internal combustion engine is operated in a predetermined rotational speed/torque region with a lean air/fuel ratio while at the same time the swirl control valves are in the effective position, while in the case of higher rotational speed/torque requirements falling outside this region the swirl control valve is brought into the starting position and at the same time the engine is operated using a substantially stoichiometric or rich mixture. However, the fuel economy is adversely affected by the stoichiometric or rich mixture.